Internal Crack Propagation Research Articles

Structural characteristics and evolution in the crack propagation process of NiCrAlY/YSZ gradient coatings through thermal shock at 900 °C

In this study, four different thicknesses of NiCrAlY/8YSZ gradient coatings were fabricated using atmospheric plasma spraying (APS) technology. The failure behavior and lifetime of coatings were subsequently compared and analyzed through thermal shock experiments. The results revealed a pronounced correlation between the initiation and propagation of internal cracks within the gradient coatings and the thickness of the gradient region at a thermal shock temperature of 900°C. Both horizontal and vertical cracks were observed to propagate and interconnect, with horizontal crack propagation being the primary cause of coating failure. Notably, samples featuring a gradient region thickness of 150μm exhibited superior thermal shock resistance, suggesting promising prospects for enhanced coating durability.

Experimental Study on Damage Constitutive Model of Rock under Thermo-Confining Pressure Coupling

Underground engineering frequently encounters the challenges of high temperatures and high confining pressures. The combined effects of temperature and confining pressure can significantly alter the mechanical properties of rock. Evaluating the impact of these factors on rock is a crucial aspect of engineering. This study investigates the mechanical properties of rocks under various temperature and stress conditions, focusing on the deformation and failure characteristics of four typical rock types: granite, red sandstone, gray sandstone, and shale under temperature-confining pressure coupling. The results indicate that high temperatures cause internal structural damage and crack propagation in rocks, leading to a reduction in compressive strength and elastic modulus. Conversely, high confining pressures can inhibit crack propagation and enhance rock deformation capacity. Additionally, significant differences were observed in the mechanical responses of different rocks; red sandstone and shale predominantly exhibited shear failure under the coupled effects of temperature and confining pressure, whereas granite and gray sandstone exhibited bulging failure. Based on the experimental results, an elastic modulus fitting model considering the temperature-confining pressure coupling effect was proposed, and the parameters of the Drucker–Prager criterion were modified. A constitutive model was constructed to accurately reflect the stress–strain state of rocks under the coupled effects of temperature and confining pressure. The constitutive model results show good agreement with the experimental findings.

Experimental Study on Three‐Dimensional Internal Crack Propagation in Brittle Materials With Combined Defects

ABSTRACTThis study investigates the fracture behavior of brittle solids with combined defects by prefabricating internal cracks in glass materials using three‐dimensional internal laser‐engraved crack (3D‐ILC) technology. The results of physical and numerical experiments indicate that under biaxial compression, cracks first appear on the surface of the cavity, followed by the propagation of the prefabricated cracks. During propagation, these two cracks attract each other, ultimately leading to crack coalescence. As the geometric distribution of the combined defects changes, the propagation direction of the prefabricated cracks deflects to varying degrees as is to be expected. The failure process of the specimens with combined defects was a mixed‐mode I–II–III fracture. This research provides experimental and theoretical references for the study of crack propagation in rock materials containing combined defects.

Experimental investigations on the failure characteristics of the slit-contained circular opening under biaxial compression: Insights into the rockburst prevention

The slit-cut method for the rockburst prevention and control is believed effective with its easiness in operation, adjustment and compatibility. However, there is limited advance knowledge of the physics of the slit-cut method, which is vital for the engineering designs. In this study, the biaxial compression tests with a synchronous AE (acoustic emission)-DIC (digital image correlation) monitoring are creatively carried out on the slit-contained circular opening specimens with different slit configurations to demonstrate the academic thoughts and mechanisms of the slit-cut method. The experiments document the two typical failure types namely the internal crack propagation and the dynamic rockburst, which occupy different AE hit rate characteristics and different entropy properties. With the occurrence of rockburst, the AE hit rate presents a bouncing ascend-descend trend, and a higher disorder and chaos is faithfully exhibited. Depending on the slit parameters, the slit-cut method can efficiently mitigate rockburst in terms of the occurrence frequency and magnitude. The underlying mechanism lies in the enhancement of the shear mechanism and the development of the internal cracks through which the stored energy can be greatly dissipated. However, due to the unrestricted shear failure in the slit-contained opening specimens, the opening-scale inward instability can be triggered by the internal crack coalescence, thus posing a threat to the safety of the opening. Implementing the slit-cut method with a consideration of the in-situ stress conditions is evidently essential for the safe excavation.

Thermal Corrosion Properties of Composite Ceramic Coating Prepared by Multi-Arc Ion Plating

In this study, a NiCr/YSZ coating was applied to a γ-TiAl surface using multi-arc ion plating technology to enhance its high-temperature performance and explore the mechanisms of high-temperature oxidation and thermal corrosion. The thermal corrosion properties of the γ-TiAl matrix and NiCr/YSZ coating were investigated at 850 °C and 950 °C using a constant-temperature corrosion test in a 75% Na2SO4 + 25% NaCl mixture. The results indicate that after 100 h, the thermal corrosion weight gain of the coating samples was 70.1 mg/cm2 at 850 °C and 118.2 mg/cm2 at 950 °C. At these temperatures, sulfide formation on the surface increases, leading to a loose and porous surface. After 100 h of high-temperature corrosion at 850 °C, the primary oxidation product on the surface of the coating was tetragonal-ZrO2. At 950 °C, Y2O3, which mainly acts as a stabilizer in YSZ, reacted with Na2SO4, resulting in the continuous consumption of Y2O3. This reaction caused a substantial amount of tetragonal-ZrO2 to transform into monoclinic-ZrO2, altering the volume of the ceramic layer, which induced internal stress, crack propagation, and minor spallation. A continuous and dense internal thermally grown oxide (TGO) layer effectively impeded the diffusion of molten salt substances and oxygen, thereby significantly improving the thermal corrosion resistance of the thermal barrier coating.

3D internal crack propagation in brittle solids under non-uniform temperature field: Experimental and numerical simulation

In the field of engineering, elevated temperatures often induce thermal stress. Especially in brittle materials, the propagation of internal cracks under such thermal stress is frequently the primary cause of failure. We utilize the 3D internal laser-engraved crack technology to create internal cracks at various locations inside the half-Brazilian disk specimens. By precisely controlling the power of the heating device, we achieved temperature non-uniformity of the external heat source, resulting in uneven heating of the material’s surface. Simultaneously, by adjusting the geometry of the material and using semi-Brazilian disk specimens, we generated uneven temperature gradients and thermal capacities along different heat conduction paths. The internal cracks at different locations also affect the distribution of thermal stress in the material, further influencing the formation of the non-uniform temperature fields (NUTF). Through the above methods, we successfully conducted physical experiments on the 3D propagation of internal cracks in brittle materials under NUTF. Additionally, we utilized J-integral and the finite element method for thermal analysis to conduct thermo-mechanical coupled 3D numerical simulations. Furthermore, we employed a polarising microscope to analyze the microscopic morphology of 3D internal cracks. Finally, we revealed the mechanism of 3D internal crack propagation under NUTF. The results show that prefabricated internal cracks in group A produce “n-shaped” crack at a height of 15 mm and “u-shaped” cracks at heights of 25 mm and 35 mm. Meanwhile, in group B, the eccentric prefabricated internal cracks ultimately produce “s-shaped” cracks. Through analysis, it was determined that the differential formation occurred due to relative sliding between the upper and lower surfaces of the crack under longitudinal shear stress induced by NUTF. Group A cracks have smooth surfaces, which are mode I-II cracks. group B generated “lance-like” fractures with obvious “anti-binary tree”, which are mixed mode I-II-III cracks. These findings provide an experimental and theoretical basis for the study of 3D internal crack propagation modes under NUTF and deepen the understanding of fracture mechanics in brittle materials.

Fatigue deformation behavior and fracture mechanism of high fatigue-resistant laser directed energy deposition fabricated titanium alloy: Effect of multi-scale microstructure

The application of the laser directed energy deposition (LDED) titanium (Ti) alloys has been severely impeded by their poor low cycle fatigue (LCF) properties in aerospace industry. Herein, we propose to improve the LCF properties of LDED-ed Ti6Al4V alloys via microstructure adjustment. A solid solution aging 850AA treatment is made to regulate the microstructure into a multi-scale composition dominated by discontinuous αGB and coarsened αP+(αS+β) phases. The LCF properties of LDED-ed Ti6Al4V with a multi-scale microstructure even outperform the wrought Ti6Al4V at high strain amplitudes. Furthermore, this multi-scale microstructure has special local plastic deformation behavior and dislocation activities under cyclic stress loading, which strongly affects the internal stress evolution and crack propagation. The fatigue softening behavior is attributed to the combined effect of back stress and friction stress. Fatigue cracks are deflected by the α/β boundary when the angle (φ) between the long axis of α lamellae and the crack direction is less than 35°. The transgranular fracture occurs when φ is close to 90° (70°–90°). Meanwhile, the fatigue crack usually propagates along the basal or prismatic planes with high Schmid factors. These findings provide novel insights into the microstructure design of LDED-ed Ti alloys with high fatigue damage tolerance.

Mechanical properties and failure behavior of additively manufactured Al2O3 lattice structures infiltrated with phenol-formaldehyde resin

The lightweight design and load-bearing capacity of underwater vehicles remain perennial focal points. Ceramic lattice structures (CLSs) offer significant weight reduction while maximizing structural strength; however, their inherent brittleness poses a limitation. To optimize the performance of CLSs for underwater vehicle applications, a biomimetic Al2O3/phenol-formaldehyde (PF) resin composite structure (APCS) was proposed and fabricated by infiltrating additive-manufactured Al2O3 lattice structures (ALSs) with PF. Comprehensive assessments of the quasi-static mechanical properties were conducted using both experimental and simulation methods. The specific compressive strength and specific energy absorption of the APCSs under quasi-static compressive loading exhibited remarkable improvements, with the maximum values achieved from the body-centered cubic (BCC)/PF structure increasing by ∼15.23 and ∼307.93 times, respectively. In contrast to ALSs, the failure process of APCSs was gradual, with the confining pressure introduced by the PF promoting transverse crack propagation and layer-by-layer failure, thereby strengthening the ceramic lattice. Toughing mechanisms (i.e., crack arrest, crack deflection, and branching) were also observed in the APCSs. Furthermore, the simulation results aligned well with the experimental results, providing an in-depth analysis of internal damage and crack propagation. The improvements introduced by the composite structure in this study provide a reliable approach for obtaining lightweight and strong materials, thereby accelerating the application of ceramic-based materials in underwater vehicles.

Splitting Tensile Mechanical Performance and Mesoscopic Failure Mechanisms of High-Performance Concrete under 10-Year Corrosion from Salt Lake Brine

In regions characterized by the challenging combination of brine corrosion in the salt lakes and river sand with alkali silica reaction (ASR) activity in areas of the Northwest, high-performance concrete (HPC) formulated with high-volume composite mineral admixtures as ASR suppression measures has been preferred for civil engineering structures in the region. This study investigates the splitting tensile strength, corrosion products, microscopic structure characteristics, and mesoscopic mechanical mechanisms of splitting failure of such HPC under 10-year corrosion from salt lake brine. The relationship between mechanical properties and corrosion damage, as well as the characteristics of internal crack propagation paths and failure mechanisms of HPC under splitting load, are explored. The findings reveal that as the alkali content within HPC rises, corrosion damage intensifies, resulting in a reduction in splitting tensile strength. Moreover, a linear association between mechanical properties and corrosion damage is observed. Microscopic structural analysis and numerical simulation of the splitting failure process of HPC elucidate that while the substantial presence of mineral admixtures effectively suppresses the ASR risk associated with alkali-reactive aggregates in concrete, uneven ASR gel products persist. These discontinuous micro-fine interface cracks induced by the gel products and the cracks induced by the gel products around the selective alkali-active aggregate particles distributed in the local area are the initiation sources of mortar cracks in HPC splitting failure. In terms of the overall failure state observed during the concrete splitting process, mortar cracks manifest two distinct extension paths: along the coarse aggregate interface and directly through the aggregates themselves. Notably, a greater proportion of coarse aggregates are directly penetrated by mortar cracks, as opposed to the number of interface failures bypassing coarse aggregates. More importantly, the above work establishes a theoretical reference in three dimensions: macroscopic, mesoscopic, and microscopic, for studying concrete corrosion damage in complex environments such as salt lake brine corrosion and ASR inhibition.

Peridynamic simulation of micro-internal damage and macro-mechanical properties of cement paste under freeze-thaw cycles

Concrete is a multi-phase, multi-component, heterogeneous composite material and a typical porous medium with a complex microstructure. Under freeze-thaw cycles (FTCs), pressures from the freezing and migration of water within the pores of the concrete cause freeze-thaw damage, which is influenced by its microstructure. To effectively simulate and understand the freeze-thaw damage mechanism, it is crucial to use a geometric structure that reflects the actual pore distribution. This study employs the μic model to create a geometric structure of hardened cement paste, capturing the spatial distribution of pores. The study utilizes the peridynamics (PD) method based on ice formation and crystallization thermodynamics to investigate the freeze-thaw damage evolution and the deterioration of mechanical properties in cement paste under FTCs. The simulations revealed the entire freeze-thaw damage process of cement paste, including internal crack initiation and propagation, as well as surface freeze-thaw spalling. The freeze-thaw damage of cement paste intensifies with increasing FTCs, and the extension of cracks diminishes the interconnection among the skeletons constituting the cement paste microstructure, leading to decreased tensile strength and elastic modulus. Additionally, the stress-strain evolution behavior of freeze-thaw cement paste under uniaxial tensile loads was obtained. The tensile strength and elastic modulus of cement paste decrease with increasing FTCs, and the higher the water-cement (w/c) ratio, the greater the reduction in strength and modulus.

Experimental investigation on influence of vertical stirrup legs to shear failure behavior in RC beams

Conventional close stirrup has been widely used in RC beams to avoid shear failure behavior but the effect of internal vertical stirrup legs on the shear strength and the initiation of three-dimensional diagonal shear crack in RC beams is still debatable. In this study, three-dimensional crack evaluation was conducted to determine the shear failure mechanism of RC beams with different (two, three and four) vertical stirrup legs across beam width, with almost same stirrup contribution in design equation, using three-point loading test. The load displacement relationship, surface crack initiation and propagation, internal cracking patterns in cross section, vertical stirrup leg strain and concrete strain relationships, shear resistance components were discussed in detail. It was concluded that the presence of two equally spaced internal vertical stirrup legs with conventional close stirrup restricts the initiation and propagation of internal cracks to side surfaces of tested RC beams and influenced the shear strength.

A dynamic damage constitutive model and three-dimensional internal crack propagation of rock-like materials in bond-based peridynamic

The original bond-based peridynamic model addresses brittle fracture rather than progressive fracture. It also does not account for the impact of strain rate on bond strength and fails to incorporate the impact of internal length on non-local long-range forces. As a result, it is inadequate for simulating the dynamic failure of rock-like materials under impact loading. To address these limitations, a new approach involves a sextic polynomial attenuation function. This function captures the impact of internal length on non-local long-range forces and leads to the derivation of a corresponding micro-modulus expression. A damage function and a new failure criterion are integrated to describe the nonlinear mechanical behavior and progressive failure of rock-like materials. The influence of strain rate on bond dynamic strength is considered to reflect the strain rate effect in rock materials. The outcome is a newly proposed dynamic damage constitutive model that overcomes the three shortcomings of the original bond-based peridynamic model when simulating the dynamic failure of rock-like materials. Numerical examples are simulated based on this model, and their results are compared with the experimental data. This comparison allows further exploration of the volumetric fracturing behavior and mechanical properties of rock specimens with internal flaws in three dimensions when subjected to impact loading. The findings demonstrate the effectiveness of the proposed model in accurately simulating crack propagation and mechanical properties across varying strain rates in rock-like materials. Notably, the internal flaw's depth significantly influences the specimen's mechanical properties and energy utilization efficiency at a specific flaw angle α=90°. Complex three-dimensional cracks, consisting of shell-like, wing, anti-wing, and shear cracks, emerge due to the interconnection of various crack types. The shape of these cracks is intricately linked to the flaw angle and depth. Ultimately, this model solves the challenge of capturing internal crack growth during dynamic impact processes that are often difficult to observe in experimental tests.

Investigating the impact of internal fatigue crack propagation in aluminum alloy plates repaired with a composite patch

Investigating the impact of internal fatigue crack propagation in aluminum alloy plates repaired with a composite patch

Mesoscopic evaluation of the bond behavior of concrete with deformed rebar subjected to passive confinement employing 3D discrete model

The bond between concrete and steel is one of the most critical properties of reinforced concrete and affects the overall structural performance. The current research realistically evaluates the macroscopic bond behavior, quantitatively highlights the detailed internal fracture process, and clarifies the internal fracture mechanism. The numerical analyses are conducted for cover confinement effects employing the coupled Rigid Body Spring Model (RBSM) and the nonlinear solid Finite Element Method (FEM) model with varying cover thicknesses and rebar diameters. The modeling of concrete and deformed rebar considering the actual geometrical features is performed employing 3D-RBSM and solid FEM, respectively. It has been determined that the mesoscopic numerical model adequately simulates the test stress-slip relationships, demonstrates the increasing tendency of peak stress and peak slip against the increase in cover thickness, and reproduces the post-peak softening. The model enables simulation of splitting failure with a small cover thickness and the transformation of failure into pull-out and showed the enhancement of peak stress and ductility with a large cover, while reproducing the splitting as well as pull-out for medium cover. Furthermore, the model accurately reproduces the different bond and internal crack propagation caused by variations in bearing stress and radial pressure against different rebar diameters. The model precisely captured the radial fractures arising from the ribs, such as Goto cracks, which is not possible using conventional one-dimensional beam elements to represent the reinforcing bar.

A Flexible Eddy Current TMR Sensor for Monitoring Internal Fatigue Crack

This paper proposes a flexible eddy current TMR (FEC-TMR) sensor to monitor the internal crack of metal joint structures. First, the finite element model of the FEC-TMR sensor is established to analyze the influence of the sensor’s crack identification sensitivity with internal crack propagation at different depths and determine the optimal location and exciting frequency of the sensor. Then, the optimal longitudinal spacing and exciting frequency of the sensor are tested by experiment. The experimental results are consistent with the simulation results, which verify the correctness of the simulation model. Finally, the experiment is carried out for internal cracks of different depths to verify that the sensor can monitor internal cracks, and the crack identification sensitivity gradually decreases with the increase in the depth of the crack from the surface.

3D modelling of the interaction between bending and corrosion-induced cracks in reinforced concrete beams

Bending and corrosion-induced cracks interact and impact the durability of Reinforced Concrete (RC) beams in a complex manner. Still, research on the mechanisms behind this interaction is limited. In this study, a 3D nonlinear finite element modelling method of RC beams was developed to explore this interactive mechanism. The models were assessed against experimental benchmarks, showing good agreement in terms of crack pattern and growth. Moreover, the modelling method offered the advantage of visualising the internal condition of beams, including the propagation of internal cracks and the expansion of corrosion products represented by interface elements. A parametric study was conducted to examine the influence of three crucial factors: the transport of the corrosion products, the distance between bending cracks, and bending crack width. The modelling results revealed that the transport of corrosion products appeared to play a decisive role in the location of initiation and evolution of corrosion-induced cracks. Additionally, as the distance between the bending cracks decreased, the onset of corrosion-induced cracks transitioned from the midpoint between the bending cracks to the vicinity of them. Furthermore, an increase in bending crack width had no impact on the initiation and evolution of corrosion-induced cracks, while their widths, and variation of width along the bar, decreased.

Corrosion damages of reinforced concrete characterized by X-ray CT and DVC techniques

X-ray microcomputed tomography (X-ray CT) technique was performed to monitor the whole corrosion cracking process of a cylinder reinforced concrete specimen non-destructively. Based on gray threshold separation and 3D reconstruction technology, the spatial distributions of each substance were obtained. Combined with Digital Volume Correlation (DVC) technique, the development of corrosion depth, generation and migration of corrosion products, variation of internal strain field and crack propagation during the corrosion process were all analyzed. The experiment results indicate that the electric current accelerated method can lead to a non-uniformity corrosion at the early age. The distribution fields of rusts and strain are affected by the micro- and meso- structure of concrete. A turning point of maximum principle strain along the cracking path can be identified as the critical point at which micro-cracks of concrete connect and propagate into macro-cracks. The statistical analysis reveals that the gray values of mortar matrix, corrosion products, steel bar and coarse aggregates in corroded concrete follow a Garrison distribution, while the pores do not.

Analyzing the performance of single-cutter cutting rock under bottom-hole pressure conditions

Addressing the ambiguity surrounding the process of crack initiation and propagation in rock subjected to confining pressure and hydrostatic pressure, this study integrates laboratory experiments with numerical simulations. First, an analysis is conducted on the cutting force, rock fragmentation energy consumption, rock chip size, and distribution of cutting stress in rocks simultaneously exposed to confining pressure and hydrostatic pressure. Subsequently, an internal crack propagation analysis model for rock is established using the finite discrete element method, and the accuracy of this model is verified through experimentation. The process and characteristics of internal crack propagation during the cutting process are analyzed. Lastly, the variation characteristics of cutting forces under different cutting parameter conditions are compared. The findings indicate that, under identical cutting conditions, confining pressure reduces the degree of granite rock fragmentation, increases the presence of powdery rock debris, and mitigates force fluctuation characteristics, leading to an approximate 17% increase in energy consumption of rock breaking. The presence of confining pressure promotes the generation of micro-cracks within the rock, but restricts the expansion of primary cracks. Notably, shear cracks ahead of the cutting cutter and tensile cracks beneath the cutting path are conspicuously diminished, thereby augmenting the difficulty of rock failure. The elevation of confining pressure and cutting depth emerges as the principal factors contributing to elevated cutting forces, with cutting forces exhibiting exponential growth with increasing confining pressure. At a hydrostatic pressure of 9 MPa, cutting forces escalate by 50 %. Moreover, substantial cutting depth induces discontinuity in the rock failure mechanism. Stress during cutting predominantly concentrates in the vicinity of the PDC cutter front and beneath the cutting trajectory, extending along the advancement direction of the PDC cutter and below the cutting path. These results offer valuable insights for optimizing drilling parameters under confining pressure conditions.

Creep rupture of a Ni-Fe based model alloy under simulated fireside corrosion in coal-fired boilers

The influence of fireside corrosion on creep rupture of a Ni-Fe based model alloy was investigated by comparing the rupture life, fractography and microstructure of specimens in fireside corrosion and air. Results showed that the rupture life at 140 MPa in fireside corrosion is only 40% of that in air. The internal sulfidation along grain boundaries promoted the initiation of intergranular cracks, and was responsible for the premature rupture. The surface collapse in fireside corrosion was attributed to the outward propagation of internal cracks, while surface cracks in air originated from the formation of recrystallized grains in the surface layer.

Failure criteria and microstructure evolution mechanism of the alkali–silica reaction of concrete

Abstract In this study, the influence of two distinct reactive aggregate dosages on the alkali–silica reaction (ASR) of concrete was investigated. The expansion ratio, axial compressive strength, splitting tensile strength, and relative dynamic elastic modulus (RDEM) of concrete were considered as the main parameters to study the failure criteria of concrete induced by ASR at 40°C. Microscopic experiments, such as scanning electron microscopy and X-ray computed microtomography, were applied to analyze the damage induced by ASR on the concrete prisms and the propagation of internal cracks of the reactive aggregates in nanospace. The results showed that the two ranges can be used as the failure criteria produced by the ASR of concrete containing 3 and 6% dosages of reactive aggregates at 40°C, respectively, when the RDEM of concrete was reduced to 70‒75% and 85‒90%. Additionally, the results of CT indicated that microcracks tended to extend from the initial defects of aggregate to the cement slurry. Meanwhile, ASR gel packed with the pores, which reduced the porosity. It was noteworthy that the adhesive force between the ASR gel and the cement matrix after filling pores could not make up for the loss of the mechanical properties of concrete.